Printing devices that create dots on printing media to give printed images have widely been used as an output device of images generated by computers and of images taken by digital cameras. The printing device creates dots at adequate densities corresponding to the tone values of image data to give a printed image. For dot formation at the adequate densities corresponding to the tone values, an object image to be printed goes through a series of image processing, which converts image data into data of dot distribution. The converted data of dot distribution ensures dot formation on the printing medium at the adequate densities corresponding to the tone values and accordingly gives a desired printed image. Diverse techniques, for example, an error diffusion method, a systematic dither method, and other various dither methods, are applicable to such image processing.
Individual dots created on the printing medium by the printing device are sufficiently small in size and are not visually recognizable in a normal state. A block of multiple dots created at close positions to one another may, however, be made rather conspicuous and undesirably worsen the printing quality. Diverse image processing techniques have been devised and proposed to attain even dot distribution, as well as to ensure dot formation at the adequate densities corresponding to the tone values of the image data. One of such techniques is disclosed in Japanese Patent Laid-Open Gazette No. 2000-261669.
The printing device successively creates dots to complete a printed image, while moving a print head relative to a printing medium. A positioning error of the print head or poor accuracy of main scan or sub-scan may result in positional misalignment of dot formation. The positional misalignment of dot formation actually creates multiple dots at close positions, regardless of the even dot distribution at the step of image processing, and thereby undesirably worsens the printing quality.
The heightened dot positioning accuracy, for example, the lowered positioning error of the print head, naturally prevents such deterioration of the printing quality. The lowered positioning error, however, remarkably increases the labor and the time required for regulation and adjustment. This time-consuming countermeasure causes other problems, that is, the raised total cost and the lowered printing speed.